Microoptical components for fiber coupling of high power laser diode bars

A microoptical beam transformation system for coupling of high power laser diode bars into multimode optical fibers is introduced. The key components are a gradient-index (GRIN) cylindrical lens for the collimation of the fast-axis radiation and an array of blazed gratings for beam deflection of the single emitter bundles in the slow-axis. Excellent optical performance and high numerical aperture is required for the cylindrical lens. The lenses are fabricated by Ag+/Na+ ion exchange and exhibit diffraction-limited performance up to N.A. = 0.5. The array of blazed gratings is produced by e-beam lithography in a variable dose writing mode. The diffraction efficiency into the desired order is higher than 85 % for each element. Using these elements coupling of more than 60 % of the output power of a 19 emitter high power laser diode bar (P-out = 20 W) into a 200 mu m fiber of N.A. = 0.2 is achieved. Applications in fiber laser pumping, solid state laser pumping as well as laser soldering and plastic welding are discussed.